Storing energy in the form of compressed gas has a long history and components tend to be well tested and reliable, and have long lifetimes. The general principle of compressed-gas or compressed-air energy storage (CAES) is that generated energy (e.g., electric energy) is used to compress gas (e.g., air), thus converting the original energy to pressure potential energy; this potential energy is later recovered in a useful form (e.g., converted back to electricity) via gas expansion coupled to an appropriate mechanism. Advantages of compressed-gas energy storage include low specific-energy costs, long lifetime, low maintenance, reasonable energy density, and good reliability.
If a body of gas is at the same temperature as its environment, and expansion occurs slowly relative to the rate of heat exchange between the gas and its environment, then the gas will remain at approximately constant temperature as it expands. This process is termed “isothermal” expansion. Isothermal expansion of a quantity of high-pressure gas stored at a given temperature recovers approximately three times more work than would “adiabatic expansion,” that is, expansion where no heat is exchanged between the gas and its environment—e.g., because the expansion happens rapidly or in an insulated chamber. Gas may also be compressed isothermally or adiabatically.
An ideally isothermal energy-storage cycle of compression, storage, and expansion would have 100% thermodynamic efficiency. An ideally adiabatic energy-storage cycle would also have 100% thermodynamic efficiency, but there are many practical disadvantages to the adiabatic approach. These include the production of higher temperature and pressure extremes within the system, heat loss during the storage period, and inability to exploit environmental (e.g., cogenerative) heat sources and sinks during expansion and compression, respectively. In an isothermal system, the cost of adding a heat-exchange system is traded against resolving the difficulties of the adiabatic approach. In either case, mechanical energy from expanding gas must usually be converted to electrical energy before use.
An efficient and novel design for storing energy in the form of compressed gas utilizing near isothermal gas compression and expansion has been shown and described in U.S. Pat. No. 7,832,207, filed Apr. 9, 2009 (the '207 patent) and U.S. Pat. No. 7,874,155, filed Feb. 25, 2010 (the '155 patent), the disclosures of which are hereby incorporated herein by reference in their entireties. The '207 and '155 patents disclose systems and techniques for expanding gas isothermally in staged cylinders and intensifiers over a large pressure range in order to generate electrical energy when required. Mechanical energy from the expanding gas may be used to drive a hydraulic pump/motor subsystem that produces electricity. Systems and techniques for hydraulic-pneumatic pressure intensification that may be employed in systems and methods such as those disclosed in the '207 and '155 patents are shown and described in U.S. Pat. No. 8,037,678, filed Sep. 10, 2010 (the '678 patent), the disclosure of which is hereby incorporated herein by reference in its entirety.
In the systems disclosed in the '207 and '155 patents, reciprocal mechanical motion is produced during recovery of energy from storage by expansion of gas in the cylinders. This reciprocal motion may be converted to electricity by a variety of techniques, for example as disclosed in the '678 patent as well as in U.S. Pat. No. 8,117,842, filed Feb. 14, 2011 (the '842 patent), the disclosure of which is hereby incorporated herein by reference in its entirety. The ability of such systems to either store energy (i.e., use energy to compress gas into a storage reservoir) or produce energy (i.e., expand gas from a storage reservoir to release energy) will be apparent to any person reasonably familiar with the principles of electrical and pneumatic machines.
In order to reduce overall pressure ranges of operation, various CAES systems may utilize designs involving multiple interconnected cylinders. In such designs, trapped regions of “dead volume” may occur such that pockets of gas remain in cylinders before and after valve transitions. Such volumes may occur within the cylinders themselves and/or within conduits, valves, or other components within and interconnecting the cylinders. Bringing relatively high-pressure gas into communication (e.g., by the opening of a valve) with relatively low-pressure gas within a dead volume tends to lead to a diminishment of pressure of the higher-pressure gas without the performance of useful work, thereby disadvantageously reducing the amount of work recoverable from or stored within the higher-pressure gas. Air dead volume tends to reduce the amount of work available from a quantity of high-pressure gas brought into communication with the dead volume. This loss of potential energy may be termed a “coupling loss.” For example, during a compression stage a volume of gas that was compressed to a relatively high pressure may remain inside the compression cylinder, or conduits attached thereto, after the movable member of the cylinder (e.g., piston, hydraulic fluid, or bladder) reaches the end of its stroke. The volume of compressed air that is not pushed onto the next stage at the end of stroke constitutes “dead volume” (also termed, in compressors, “clearance volume”). If the volume of high-pressure gas within the cylinder is then brought into fluid communication (e.g., by the opening of a valve) with a section of intake piping, a portion of the high-pressure gas will tend to enter the piping and mix with the contents thereof, equalizing the pressure within the two volumes. This equalization of pressure entails a loss of exergy (i.e., energy available as work). In a preferred scenario, the gas in the dead volume is allowed to expand in a manner that performs useful work (e.g., by pushing on a piston), equalizing in pressure with the gas in the piping prior to valve opening. In another scenario, the gas in the dead volume is allowed to expand below the pressure of the gas within the intake piping, and pressure equalization takes place during valve transition (opening).
In another example of formation of dead space in a CAES system during an expansion procedure, if gas is to be introduced into a cylinder through a valve for the purpose of performing work by pushing against a piston within the cylinder, and a chamber or volume exists adjacent to the piston that is filled with low-pressure gas at the time the valve is opened, the high-pressure gas entering the chamber is immediately reduced in pressure during free expansion and mixing with the low-pressure gas and, therefore, performs less mechanical work upon the piston than would have been possible without the pressure reduction. The low-pressure volume in such an example constitutes air dead volume. Dead volume may also appear within the portion of a valve mechanism that communicates with the cylinder interior, or within a tube or line connecting a valve to the cylinder interior, or within other components that contain gas in various states of operation of the system. It will be clear to persons familiar with hydraulics and pneumatics that dead volume may also appear during compression procedures, and that energy losses due to pneumatically communicating dead volumes tend to be additive.
Moreover, in an expander-compressor system operated to expand or compress gas near-isothermally (i.e., at approximately constant temperature) within a cylinder, gas that escapes the cylinder to become dead volume (e.g., by displacing an incompressible fluid) in a hydraulic subsystem may, as pressures change within the system, expand and compress adiabatically (i.e., at non-constant temperature), with associated energy losses due to heat transfer between the gas and materials surrounding the dead volume. These thermal energy losses will tend to be additive with losses that are due to non-work-performing expansion of gas to lower pressures in dead volumes.
Therefore, in various compressor-expander systems, including isothermal compressor-expander systems, preventing the formation of dead volume will generally enable higher system efficiency. Attempts to minimize dead volume frequently involve reducing the sizes and lengths of conduits interconnecting cylinders and other components. However, such efforts may not eliminate all dead volume and tend to constrain the overall geometry and placement of individual system components. Therefore, there is a need for alternative or additional approaches to reducing dead volume and/or the deleterious effects of dead volume in pneumatic components in order to reduce coupling losses and improve efficiency during compression and/or expansion of gas.